Hi Iraida, I am sending my answer to the mailing list -- in part because it is an interesting and relevant question and in part because I am not sure I know the correct answer to your question. On Wednesday 17 May 2006 08:02, you wrote:

Dear Bruce,

After reading on site http://millenia.cars.aps.anl.gov/pipermail/ifeffit/ about possible differences in calculations by FEFF6 and FEFF8 I've tested my models by both FEFF programs. And now, perhaps I'm making a stupid mistake in atom.inp file and next in feff.inp files.or I have a problem. I need to examine GaxMn1-xAs samples with the concentration of Mn about 1-2% (for example). The measurements were done for K edge of Mn. Within the initial model Ga atoms are replaced by Mn. My atom.inp file is present below:

title = GaAs:Mn (Mn_Ga model) space = F -4 3 M a = 5.65330 core = Ga edge = K rmax = 6.0 out = feff.inp atoms ! elem x y z tag Ga 0.00000 0.00000 0.00000 As 0.25000 0.25000 0.25000

Then in the feff.inp file in "potentials" card I change the central Ga atom to Mn (in connection with the small concentration of Mn (1-2%) I didn't add any Mn atoms in the second and third coordination shells). For this model in the elementary cell we have: 48.4372% of Ga atoms; 50% of As atoms and 1.56% of Mn atoms. Our cluster is limited to 6 Å (rmax = 6.0).

POTENTIALS * ipot z [ label l_scmt l_fms stoichiometry ] 0 25 Mn -1 -1 0 1 33 As -1 -1 1 2 31 Ga -1 -1 1

ATOMS 0.00000 0.00000 0.00000 0 Mn 0.00000 -1.41332 -1.41332 1.41332 1 As 2.44795 -1.41332 1.41332 -1.41332 1 As 2.44795 1.41332 -1.41332 -1.41332 1 As 2.44795 1.41332 1.41332 1.41332 1 As 2.44795 -2.82665 0.00000 -2.82665 2 Ga 3.99749 2.82665 -2.82665 0.00000 2 Ga 3.99749 0.00000 -2.82665 2.82665 2 Ga 3.99749 -2.82665 -2.82665 0.00000 2 Ga 3.99749 0.00000 2.82665 2.82665 2 Ga 3.99749 0.00000 2.82665 -2.82665 2 Ga 3.99749 2.82665 0.00000 2.82665 2 Ga 3.99749 -2.82665 0.00000 2.82665 2 Ga 3.99749 0.00000 -2.82665 -2.82665 2 Ga 3.99749 -2.82665 2.82665 0.00000 2 Ga 3.99749 2.82665 0.00000 -2.82665 2 Ga 3.99749 2.82665 2.82665 0.00000 2 Ga 3.99749 .......... ..........

Then I do the calculations by FEFF6 and FEFF8 programs, respectively.

In the table you'll find the differences in Amp for calculations done by FEFF6 and FEFF8 programs (there are values only for the first three single scat.).

# degen reff Amp Scat. path 1 4 2.448 100 feff6 Mn-As 100 feff8 2 12 3.998 88.07 feff6 Mn-Ga 101.44 feff8 5 12 4.688 64.42 feff6 Mn-As 67.22 feff8

My question is: why do we have such big differences in Amp? Should I prefer the calculations done by FEFF6 program?

Asimilar question to this came up recently on the mailing list: http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2006-March/002368.html And here is my response, explaining my best memory of how that amplitude term is computed http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2006-March/002405.html Jeremy and Feng also give useful responses to the original question in that thread. The bottom line is, I think, that the amplitude term is a very imprecise measure of the actual size and importance of a path. What's more, the exact details of how it is inaccurately calculated may be different in Feff6 and Feff8 (although I do not know that for a fact). What seems like a much more relevant comparison between the two is how they actually compare when plotted. So, import both the feff6 and feff8 calculations into Artemis and compare the two calculations path by path in k-space AND in R-space. When you do that, how different are they? And where in k- and R-space are the differences most pronounced? That will, I think, be a much more accurate comparison of the two calculations. Hope that helps, B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/